Under EU pressure, Visa and MasterCard dropped their interchange rate (the rate paid to to card issuing bank) dramatically in 2015 . That’s why cashback Visa and MasterCard credit cards disappeared.
What’s left is the merchant service fee and the scheme processing fee. Both are very low. The merchant service fee is the part that varies on business size – there’s less cost in signing one deal with Tesco than 100,000 with small businesses.
The schemes have also moved away per transaction fee cap for debit – it got a bit ludicrous when merchants were being charged the same for selling a 50p packet of crisps and a £50,000 car.
Amex were originally not covered by the EU ruling due to small market share but are now included.
@nickdavies figures look about right.